1. Field of the Invention
The present invention relates to a method of manufacturing a packaged soybean curd with long shelf life of at least one month at room temperature and without inclusion of any artificial additives such as coagulating agents, germicides and the like.
2. Description of the Prior Art
A soybean curd called "TOFU" has been well known in Japan for a long time. Since soybean curd contains high quality protein and is of low price, it has come to be attractive and highly regarded in many countries in the world.
In fact, in recent years, soybean curd has been utilized as one of the materials for making seasonings or prepared foods such as dressings, spreads, sauces, hors d'oeuvres, salads and the like not only in Southeast Asia but also in Western countries.
In this regard, the development of glucono-delta-lactone as a coagulating agent for soybean juice has made a considerable contribution, since glucono-delta-lactone moderately reacts with soybean juice compared to the traditionally used coagulating agents such as bittern (so called "Nigari" in Japan) and an agent called "SUMASHIKO" in Japan and comprised of calcium sulfate. In fact, glucono-delta-lactone makes it possible to manufacture soybean curds having uniform quality on a mass production scale, since glucono-delta-lactone can be sufficiently mixed with soybean juice prior to commencement of the coagulation reaction at low temperature. Also mass production of soybean curds in an aseptic atmosphere makes it possible to prolong shelf life of soybean curds without inclusion of any germicide.
Some of the typical prior art relating to mass production of soybean curds having long shelf life are disclosed in Japanese Patent Public Disclosures No. 50(1975)-160450, No. 51(1976)-15657 and No. 51(1976)-73159. In these references, glucono-delta-lactone is used as a coagulating agent and a divalent metal salt is used as a supplementary agent.
However, reflecting the recent trend of the public toward natural foods, it has been proposed to provide a method for producing a packaged soybean curd containing no artificial additives such as artificial coagulating agents and synthetic germicides.
One known method for producing such a soybean curd without inclusion of any artificial additives is disclosed in Japanese Patent Publication Gazette No. 53(1978)-25015 wherein a certain amount of water and oil (or fat) are added to and homogeneously mixed with soy protein isolate having a specific range of gel-formation capability and then the resulted mixture is heated to effect coagulation. In this method, however, there are some defects that the material is limited only to those soy protein isolates having a specific range of gel-formation capability. In addition it is difficult to obtain uniform quality products because the solubility of soy protein isolate deteriorates during storage. Another problem is that soy protein isolate may coagulate by heating only when it is dissolved in higher concentration, but if the concentration is increased the viscosity of the solution is increased and this makes homogeneous coagulation difficult.
Soybean cheese and soybean yogurt prepared by lactic acid fermentation of soybean juice have also been known. However, as will be described in Test 1, soybean juice does not form a curd having the texture of TOFU following lactic acid fermentation of soybean juice. As far as the inventors know, there is no prior reference which clearly states or suggests a method for producing a soybean curd by subjecting soybean juice to lactic acid fermentation and then to thermal coagulation.